One measure of infidelity among couples is the frequency of children secretly conceived with a different partner, leading to "non-paternities".
Such covertly illegitimate children amount to about 1–2% of newborns in studied populations.
Sex ratio theory is a theory that explains the relationship and sexual dynamics within different areas of the world based on the ratio of the number of marriage-aged men to marriage-aged women.
According to this theory, an area has a high sex ratio when there is a higher number of marriage-aged women to marriage-aged men and an area has a low sex ratio when there is more marriage-aged men to marriage-aged women.
In terms of infidelity, the theory states that when sex-ratios are high, men are more likely to be promiscuous and engage in sex outside of a committed relationship because the demand for men is higher and so this type of behaviour, which is desired by men, is more accepted.